Gas exchange with a reflecting system for inhalational anaesthesia

Abstract: In order to reduce comsumption of inhalational anaesthetsics during high flow anaesthesia a system open in regard to oxygen, nitogen and nitrous oxide, but closed to inhalational anaesthetics, was developed. This was achieved by a reflecting filter for inhalational anaesthetics made of active carbon. The principle has been developed into the commersially available Anesthetic Conserving Device (ACD, AnaConDa®).The ACD is a modified heat- and moisture exchanger (HME) containing av bacterial and viral filter as well as a carbon filter. The ACD can be used during surgery for administering the inhalational agents isoflurane and sevoflurane as an alternative to low-flow anaesthesia systems. It can also be used in intensive care units to administer sedation using isoflurane or sevoflurane to critically ill patients. The thesis is based on four papers. In paper I it is shown that that the wash-in kinetics for sevoflurane delivered by the ACD is similar to a vaporiser. With the ACD, end-tidal sevoflurane accurately reflects arterial sevoflurane tension whereas inspired tension may be underesteimated. Despite an increase in tidal volume of patients in a group with an ACD corresponding to the larger internal volume of the device compared to a conventional HME, the PaCO2 was considerably higher in this group. In paper II CO2 exchange and airway dead space is compared using an HME, ACD without reflecting filter and ACD with reflecting filter in a laboratory set-up. It is shown that the ACD binds CO2 resulting in a dead space effect 180 ml in excess of its internal volume. This is due to adsorption of CO2 in the ACD during expiration and return of CO2 during the following inspiration. In paper III the ACD is used without inhalational anaesthetics in patients as well as in a laboratory set-up with varying temperature and humidity. It is confirmed that an ACD used in humans as well as in a test lung increases the dead space effect and CO2 rebreathing to a greater extent than can be explained by its internal volume. This is caused by adsorption of CO2 in the ACD during expiration and release of CO2 during inspiration. It is also shown that rebreathing of CO2 is attenuated but not abolished by humidity. In paper IV the ACD is used with sevoflurane in humans and in a laboratory set-up. It is shown that sevoflurane attenuates but does not abolish rebreathing of CO2, thereby reducing the dead space effect of the ACD. In conlusion, the ACD causes a dead space effect larger than its internal volume due to CO2 rebreathing. This may influence the clinical use of the ACD.